Skeletal muscle has a remarkable ability to grow and repair throughout life. Deficiencies in skeletal muscle regeneration contribute to normal human aging and muscular dystrophies. Understanding both the process of how muscle senses damage and the molecular details of how muscle is repaired are of central importance in human biology. We believe that we have identified a novel connection between the inflammatory response and skeletal muscle remodeling mediated by the myogenic regulatory family (MRF). The inflammatory response plays an important role in initiating skeletal muscle regeneration. One of the cytokines involved in the inflammation response is the anti-fibrotic agent interferon gamma (IFN-?). IFN-? has complex effects on myogenesis, but is required for efficient muscle regeneration in vivo. We have identified the class II transactivator, CIITA, as a factor that interacts with myogenin, the MRF that mediates terminal differentiation. In immune cells, IFN-? stimulates both the activation and repression of a large set of genes through the action of CIITA, which serves as both a co-activator and co-repressor. CIITA is activated by IFN-? through the JAK/STAT pathway which leads to the direct activation of CIITA by STAT1. CIITA has both constitutive and IFN-? stimulated roles in the cell. We have confirmed that CIITA is expressed in the C2C12 myoblast line and shown that CIITA acts as a specific inhibitor of myogenin's activity as a transcriptional activator of muscle specific genes. CIITA expression is stimulated by IFN-?, and both stimulation with IFN-? and over expression of CIITA down regulate muscle specific gene expression and inhibit differentiation. Thus, we hypothesize that CIITA is the mediator of the IFN-? signal in skeletal muscle cells. In this proposal, we seek to understand how CIITA contributes to myogenesis and muscle repair. First, we propose to extend our current work on the role of CIITA in myoblasts by confirming our preliminary findings in primary myoblasts and confirming the anti-differentiation effect of CIITA with over expression and knock down studies. We also plan to initiate studies on the role of IFN-? in normal myogenesis by characterizing the level of IFN-? produced by myoblasts and by blocking the IFN-? receptor to assay the effects of loss of IFN-? signaling. Next, we seek to understand the contribution of CIITA to muscle repair by conducting muscle damage studies on mice with a disruption of the CIITA allele. Finally, we seek to determine if CIITA is the mediator of IFN-? and STAT1 signaling in myoblasts. Our preliminary data suggests that CIITA and its inhibition of myogenin are responsible for many of the effects of IFN-? and STAT1 on myogenic cells. We will attempt to support our hypothesis by confirming that STAT1 activates CIITA in myoblasts and by using STAT1 inhibitors to confirm a loss of anti differentiation effects following IFN-? stimulation. We will then attempt to rescue this effect by over expressing CIITA. We will also assay for IFN-? effects in Myog-/- myoblasts to determine if myogenin is required to mediate the IFN-? mediated down regulation of muscle specific targets. PUBLIC HEALTH RELEVANCE: Skeletal muscle is able to grow and repair throughout life. When muscle cannot be repaired due to age or muscular dystrophies, this leads to a loss of the ability to move and eventually leads to death. This proposal seeks to characterize a novel link between the signaling induced by the inflammatory response to muscle damage and the factors that can repair muscle fibers. We have discovered a factor in skeletal muscle cells that both responds to the signals released from inflammation and represses the activity of a specific factor required for the final steps in rebuilding muscle. We propose that this system gives muscle the time it needs to initiate repair before committing to the final steps of repair. This proposal seeks to further characterize this factor in muscle cells and determine its cellular role in skeletal muscle differentiation and repair. This work has important implications in understanding how skeletal muscle repairs itself after injury and may be able to offer novel insight into how to stimulate muscle regeneration in the case of aging or disease.